A wearable device refers to an intelligent mobile device which can be directly worn by a human body or can be integrated in clothes, accessory, or the like, and can record data such as data of human body. For example, the wearable device can be a smart band, a smart watch, or the like.
Because the size of a wearable device is small, the battery capacity is small. Therefore, the battery life of the wearable device is poor, and frequent charging is needed (for example, charging one time or many times per day). Generally, the wearable device is charged through the following method: converting an alternating current (for example, 220V or 110V) into a direct current (for example, 5V) by a separated charging device, and then charging the battery with the direct current. However, charging through the above method is not convenient because frequent charging is needed for the wearable device. Therefore, in order to address the issue of charging inconvenience, the wearable device equipped with a solar cell is also provided now. However, the charging efficiency of the wearable device equipped with the solar cell is low because of the adverse effect of the ambient stray light, and therefore, it's not possible to fully charge the battery of the wearable device in a very short time.